The invention relates to an electrodeless low-pressure sodium vapour discharge lamp comprising
a discharge vessel which is closed in a vacuumtight manner, has an internal surface, and contains a filling comprising sodium vapour and rare gas, which discharge vessel comprises an enveloping part and a sunken part, which parts are interconnected at a first end of the discharge vessel, PA1 a body of soft magnetic material which is surrounded by an electric coil and which is provided together with said coil in the sunken part of the discharge vessel, PA1 an evacuated outer bulb in which the discharge vessel is accommodated.
Such a lamp is known from EP 0 298 538 A1 which corresponds to U.S. Pat. No. 4,922,157. Such a lamp is attractive inter alia because the discharge vessel has small dimensions for a given power compared with the discharge vessel of a conventional low-pressure sodium vapour discharge lamp, which is a tube bent into a U-shape having a seal at either end around an electric lead-through to an electrode. The light generated by an electrodeless lamp, therefore, can be better concentrated into a beam by means of a luminaire.
It has been found that construction glasses, i.e. glass types which have good mechanical properties and are easily processable on an industrial scale, for example lime glass, are not resistant to sodium vapour. To realize a sufficiently long operating life, the discharge vesel of the conventional lamp is accordingly made from a tube of construction glass which has on its inner surface a layer of the sodium-resistant borate glass. It is not possible in practice to manufacture the tube entirely from borate glass since this glass is strongly hygroscopic.
Such a conventional layered tube is made in a drawing process in which softened borate glass and softened construction glass flow through a first and a second circumferential slot, respectively, the first slot being situated concentrically inside the second slot. During this, a moisture-free gas flows through an opening situated centrally relative to the slots, which presses the inner layer of borate glass against the outer layer of construction glass. Since the gas is moisture-free and since the borate glass is surrounded by a shell of construction glass, a reaction of the borate glass with water is precluded. During storage and transport, reaction between the borate glass and water can be avoided in that the tubes are filled with a moisture-free gas and stoppered at the ends.
The enveloping part of the discharge vessel of an electrodeless low-pressure sodium vapour discharge lamp may be manufactured from a conventional layered tube.
In contrast to the enveloping part, the sunken part will find its outer surface in contact with sodium vapour. To protect also the sunken part against sodium vapour, therefore, it is necessary for the outer surface thereof to have a protective layer.
A drawing process in which glass tubing is made with a layer of borate glass at the outer surface has the disadvantage that reaction of the borate glass with water can only be prevented when the entire ambience in which the drawing process takes place is free from moisture. Reaction of the borate glass layer with water must also be avoided during storage and transport of such tubes.
To form the sunken part from such a layered tube it is necessary to close an end of the tube by sealing. If the tube is heated for this purpose from the outside, there is a risk that the borate glass contracts into drops in locations at the outer surface having a comparatively high temperature, so that portions of the tube no longer have a protective layer. It is true that the temperature profile in the borate layer can be more even in the case of heating of the tube from the inside, but this imposes stringent restrictions on the geometry of the heat source. In addition, this involves the risk that not only the end, but also other portions of the tube are softened, so that heat source gets jammed in the tube, or the body of soft magnetic material and the coil no longer fit in this tube.